This invention relates to a method of forming a plastic article and more particularly to a method of forming a deep draw twin sheet plastic article.
Twin sheet plastic articles are typically formed by thermoforming an upper plastic sheet over a male mold having a downwardly extending protrusion, thermoforming a lower plastic sheet over a female mold having a cavity generally complementary to the male mold protrusion, and thereafter selectively fusing the thermoformed upper and lower sheets together with the upper sheet nested within the lower sheet to form the twin sheet structure. Whereas this twin sheet structure provides, for a given quantity of plastic, an extremely strong and durable article, problems arise when an attempt is made to apply the twin sheet technology to articles having a configuration requiring a deep drawing of the plastic.
Specifically, when the sheets are thermoformed over an upper male protrusion and into a lower female cavity, the plastic material tends to thin out with the thinning maximizing in the upper corner region of the male protrusion and the lower corner region of the female cavity so that the resulting twin sheet article has thinned and weakened regions. In an effort to overcome this problem, it has been proposed to utilize thicker plastic sheets so that, despite the thinning, the final twin sheet article still has satisfactory strength. However, this solution results in a heavier article for a given strength specification as well as a more expensive article.